Digital Subscriber Line or Loop (DSL) communication technologies have been adopted by telephone service providers as a way of extending digital service to customer premises (CP) such as homes and offices. The advent of digital communication technology has resulted in an evolutionary change to communication systems as the facilities of switches and trunks in the networks of telecommunications service providers were converted first from analog to digital. Next, consumers wanted digital access to these digital capabilities in the network facilities of service providers. However, delivering digital services over the local loop or subscriber line facilities to cover what is often colloquially called “the last mile” to the customer premises has been more of a challenge to provision. While various mechanisms have been used to deliver digital services to customer premises, making major changes to the wiring plant that feeds subscribers generally is still prohibitively costly. For customers located near a central office (CO) or close to a digital loop carrier (DLC) system, with the generally corresponding short cable wiring runs, DSL service is often available.
However, DSL capabilities still are not available to many customers located at farther reaches from central office switches and/or digital multiplexers such as a DLC. Furthermore, the historical telephone wiring plant feeding many customer locations was designed and optimized for the analog voice frequency communications of plain old telephone service (POTS) primarily found in the 0 to 4 KHz range. (One skilled in the art will be aware that the common bandwidth for unloaded POTS loops is primarily found in the 0 to around 4 KHz range, while the common bandwidth for loaded POTS loops is primarily found in the 0 to around 3.4 KHz range. One skilled in the art will be aware of these actual bandwidth differences of loaded and unloaded loops in carrying native POTS communication even though the POTS baseband is commonly referred to as a 0-4 KHz POTS baseband. One of ordinary skill in the art will be aware that such a reference is not completely accurate for loaded loops, but is a useful shorthand when discussing the POTS baseband configurations.)
Historically, telephone companies often found it advantageous to install inductors or load coils on many local loops to optimize performance of the loops in carrying POTS voice communication. Generally, the load coils or inductors were installed in series at various points along the telephone local loop. On a properly designed local loop, load coils generally are placed on subscriber loops that are greater than or equal to 18 Kft. in length. The load coils commonly used by the Regional Bell Operating Companies (RBOCs) have 88 milli-Henrys as the standard nominal inductance value for the coils. In general, load coils are spaced along a subscriber loop beginning at approximately 3 Kft. from a line card in a CO switch or DLC chassis with additional coils generally spaced along the loop approximately each 6 Kft. thereafter. The customer end portion of a local loop generally is allowed to have lengths ranging from 3 Kft. to 12 Kft. beyond the last load coil. In general, the local loop design rules used by the RBOCs specify that three or more load coils should be used on loops that are 18 Kft. or longer in length. In some special assembly situations, such as but not limited to analog POTS loops used as trunks for a customer's PBX, the RBOCs may use load coils on loops as short as 15 Kft. in length with a minimum of two load coils.
Essentially, adding an inductor in series results in the creation of a low-pass filter. While the low-pass filtering of these load coils improves performance in the 0 to around 3 KHz base bandwidth of an analog POTS interface, the filtering results in detrimental effects (primarily attenuation) on the higher frequency signals above 3 KHz that generally are used in DSL technologies. Unfortunately, the problem is not solved simply by getting the service provider to remove the load coils on each loop. While such an action certainly solves the technical limitations of load coils on DSL performance, economically it is an expensive process to remove the load coils. Furthermore, removal of the loading coils re-introduces the voice-band degradations that the coils were introduced to overcome. As a result, the service provider often cannot justify the costs of basically custom re-engineering each of the multitude of subscriber lines to remove load coils in order to earn the additional revenues from offering DSL service. Removing load coils generally would involve identifying the location of all of the load coils on a subscriber loop and sending a technician to each location to take the load coil out of the subscriber line circuit. Just sending the technician to each location would be costly enough. However, the physical process of removing load coils can create additional problems. For instance, most cables in the underground are pulp insulated such that wire pairs can be easily damaged as a result of a technician or cable splicer working on the splice to locate the wire pair affected by load coils. Obviously, damage to other pairs may knock out phone service to existing customers.
In addition, often the databases and records of service providers are incomplete and/or inaccurate in keeping up with the location of all the load coils that were installed on a particular subscriber loop over the years. Thus, in some cases various transmission line tests (such as, but not limited to tests performed by a time-domain reflectometer or TDR) might have to be performed to determine the distance along a subscriber loop transmission line at which there are changes in the characteristic impedance of the transmission line indicating potential items such as, but not limited to, load coils, junction splices, bridge taps, and/or connection points.
Because an impedance mismatch in a transmission line causes at least part of the energy from propagating electromagnetic signals to be reflected or echoed back in the opposite direction of the original propagation, a TDR and other types of test equipment generally can be used to send signals down a transmission line and measure the amount of time before a signal reflection or echo is received at the test equipment. This time measurement together with the estimated speed of propagation of the electromagnetic wave in the transmission line medium can be used to provide an estimate of the distance along the transmission line (such as a subscriber loop) where impedance mismatches occur. In general, telephone companies (or telcos) maintain computerized or paper plat records showing the location of telco facilities such as, but not limited to, wires, splice points, cross-connects, and DLCs used in delivering service to residential and commercial areas. The transmission line distances provided by a TDR or other test equipment for the potential location of impedance mismatches, which might be caused by load coils, would have to be used to estimate the approximate geographic location of a load coil based on the potentially inaccurate service provider records showing the wiring path for the transmission line from the central office or DLC to the customer premises. Obviously, such activities of identifying load coils and possibly having a technician physically track down the path followed by a subscriber loop transmission line can b e costly. As a result of these load coil issues, either some customers are not offered DSL service at all or the price of the service is higher than it should be because of the increased costs of removing load coils. Thus, service providers are not able to offer DSL service to a relatively larger number of potential subscribers because of the load coil issue. Improving this load coil problem would increase the number of customers and associated revenues available to the service provider.
In addition, subscriber loops normally run through various other facilities in connecting a customer premises to a line card in a central office switch or in a digital multiplexer such as a DLC. Often telephone wiring is run in groups of large multi-pair cables from a connection co-located with the line cards to a splice point, junction terminal, or cross-connect point. The cross-connect point generally is an unpowered box where technicians can cross-connect the wires leading to a customer premises with the appropriate wires leading back to the line cards in a switch or DLC. Often the portion of a local loop transmission line from a cross-connect box back to a line card is known as the F1 or feeder portion of a local loop, while the portion of a local loop transmission line from the cross-connect to the customer premises is known as the F2 or distribution portion of a local loop. Normally, the cross-connect box uses various mechanical technologies (such as but not limited to various punch-down block technologies) that are common in telephone wiring to simplify a technician's work in connecting the two portions of a subscriber loop. Unlike a digital loop carrier (DLC) cabinet, which generally is provided with power from the central office (and/or other sources) to enable the operation of the electronic devices of the line cards and multiplexing equipment, cross-connect boxes and/or cabinets generally are not provided with power other than the powering delivered over the POTS interface of each in-service POTS loop that provides for basic POTS functionality powering to a customer premises. This power on a POTS loop is designed for powering POTS analog phones with basic functionality (such as, but not limited to, dial tone) at the customer premises and generally does not provide a significant amount of excess power that could be siphoned off to power other types of electronic digital communications equipment. Often analog phones with POTS interfaces that offer more functionality such as a speaker phone or memory need additional power from an AC outlet or battery at the customer premises because the POTS interface does not provide enough power to meet the needs of these additional electronic functions.
In providing DSL service, often the network-side or CO-side of the DSL line is terminated in a DSLAM (Digital Subscriber Line Access Multiplexer) that usually is capable of supporting multiple DSL loops. One skilled in the art will be aware that a DSLAM normally comprises a plurality of DSL modems and some statistical multiplexing concentration equipment. However, such DSLAM equipment normally needs a reasonable amount of power and is usually placed in locations where power is readily available such as a central office (CO) or DLC cabinet. As cross-connect boxes generally do not have power available for powering active electronics, DSLAMs are not placed in cross-connect boxes. Furthermore, cross-connect boxes generally are not large enough to encompass significant amounts of additional electronic equipment in contrast to the relatively larger cabinets containing DLCs. Thus, normal deployment of DSLAMs for providing DSL service to customers does not place DSLAMs in cross-connect boxes at least because cross-connect boxes generally do not have a ready source of sufficient power and cross-connect boxes generally are not large enough for holding the DSLAM equipment.
Given these and other limitations of the wiring cable plant that was often originally installed many years ago to just provide basic POTS, new innovations that increase the availability and lower the total costs of delivering digital subscriber line (DSL) service provide benefits that can allow more consumers to obtain a reasonable digital service access line at an affordable price point.